The present invention relates to a load drive control circuit for controlling the driving of a load such as a motor, a lamp, or the like.
In the prior art, a motor driver, such as a motor drive circuit, is used for driving a motor. Patent document 1 describes an H-bridge type motor drive circuit. FIG. 6 shows an H-bridge type motor drive circuit 81. The motor drive circuit 81 includes a first bridge circuit 83, which is connected to a first terminal M1 of a motor 82, and a second bridge circuit 84, which is connected to a second terminal M2 of the motor 82. The first bridge circuit 83 includes two series-connected transistors 85 and 86 connected in series. A node or an output terminal between the transistors 85 and 86 is connected to the first terminal M1 of the motor 82. The second bridge circuit 84 also includes two series-connected transistors 87 and 88. A node or an output terminal between the transistors 87 and 88 is connected to the second terminal M2 of the motor 82.
As shown in FIG. 7(a), the potential at the output terminal of the first bridge circuit 83 is high when the transistor 85, which is closer to the power supply Vcc, is activated and the transistor 86, which is closer to ground GND, is deactivated. In this state, the bridge output of the first bridge circuit 83 is high. As shown in FIG. 7(b), the potential at the output terminal of the first bridge circuit 83 is low (bridge output is low) when the transistor 85 is deactivated and the transistor 86 is activated. As shown in FIG. 7(c), the first bridge circuit 83 is in a high-impedance state (Hi-Z state) when the two transistors 85 and 86 are both deactivated. The operation of the second bridge circuit 84 is similar to the first bridge circuit 83.
In the situation shown in FIG. 8(a), the first bridge circuit 83 is in a high state, and the second bridge circuit 84 is in a low state. In this case, current flows from the activated transistor of the first bridge circuit 83 to the activated transistor of the second bridge circuit 84. As a result, the motor 82 is driven to produce forward rotation. In the situation shown in FIG. 8(b), the first bridge circuit 83 is in a low state, and the second bridge circuit 84 is in a high state. In this case, current flows from the activated transistor of the second bridge circuit 84 to the activated transistor of the first bridge circuit 83. As a result, and the motor 82 is driven to produce reverse rotation.
The motor drive circuit 81 is switched to a brake mode as shown in FIG. 8(c) or FIG. 8(d) to stop rotation produced by the motor 82. The two bridge circuits 83 and 84 are both in a low state in the brake mode of FIG. 8(c). The two bridge circuits 83 and 84 are both in a high state in the brake mode of FIG. 8(d).
A power supply leakage may occur for reasons such as condensation or a wiring abnormality depending on the usage state of the motor drive circuit 81. An example of a problem that may occur due to power supply leakage when switching both of the bridge circuits 83 and 84 to a low state to stop the rotation produced by the motor 82 will be described with reference to FIG. 9. In this example, the power supply leakage forms a power supply leakage resistor 89, and the first terminal M1 of the motor 82 is short circuited with the power supply Vcc due to such power supply leakage resistor 89. This is an accidental short circuit to a higher-potential point, or the power supply Vcc. Such a short circuit is herein referred to as higher potential point short circuit. Such a short circuit may cause overcurrent to flow from the power supply Vcc to the activated transistor (in FIG. 9, the transistor 86) in the first bridge circuit 83. This may damage the activated transistor.
A current detection circuit for detecting the flow of current to the motor drive circuit 81 (bridge circuits 83 and 84) or a voltage detection circuit for detecting the voltage applied to the motor 82 is used to protect the transistors of an H-bridge type motor drive circuit so that they are not damaged by overcurrent. A detection signal of the current detection circuit or voltage detection circuit is provided to a CPU. The CPU monitors the value of the current flowing to the motor drive circuit 81 based on the detection signal. When a higher potential point short circuit is detected, the CPU deactivates the transistors 85 and 86 in the bridge circuit (in FIG. 10, the first bridge circuit 83) located at the side in which the higher potential point short circuit occurred. Further, the CPU shifts the bridge circuit 83 located at the side in which the higher potential point short circuit occurred to a Hi-Z state, as shown in FIG. 10. Thus, the transistor 86 of the first bridge circuit 83 in which the higher potential point short circuit occurred is less likely to be damaged by overcurrent.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2006-158162